The present invention relates to an electromagnetic proportional actuator for use in an electromagnetic proportional control valve and the like.
It is required for an electromagnetic proportional actuator incorporated in an electromagnetic proportional control valve or the like, to accurately displace a plunger in response to exciting electric current passing through a solenoid, which plunger is for controlling pressure for opening the valve. In view of such requirement, as shown in FIG. 1, an electromagnetic proportinal actuator of the prior art in general comprises a housing 1 having formed therein a cylindrical portion 2. A plunger 4 is received in the cylindrical portion 2 and is fixed mounted on a shaft 3 formed of a non-magnetic material for movement therewith. The shaft 3 has opposite ends thereof axially slidably supported by a pair of bearings 5 and 6, respectively. A slight clearance is defined between an inner peripheral surface of the cylindrical portion 2 and an outer peripheral surface of the plunger 4 to prevent the plunger 4 from being electromagnetically attracted to the inner peripheral surface of the cylindrical portion 2. The plunger 4 is controlled in axial displacement in response to exciting electric current passing through a solenoid 7 wound around an outer peripheral surface of the cylindrical portion 2.
With the electromagneitc proportional actuator arranged as described above, however, such problems arise that since the actuator requires the shaft 3 and the bearings 5 and 6, the actuator has a great number of items or components and is complicated in structure. In addition, it is necessary to maintain tight machining tolerances of the bearings 5 and 6 supporting the opposite ends of the shaft 3, as well as those of the cylindrical portion 2 and the plunger 4, the latter being to maintain required accuracy of the clearance between the inner peripheral surface of the cylindrical portion 2 and the outer peripheral surface of the plunger 4. Therefore, the prior art actuator is inevitably expensive and large in size.